No. You mean if primary database is in archived mode then standby alos will be archived mode. Your standby database may or may not be in archived mode since there is one to one DR you have established. Your primary database must be in Archived log mode otherwise what is the use of creating DR.

3) A primary DB can maintain lot of standby databases. So DB_UNIQUE_NAME init parameter seperates which standby is maintained for LOG_ARCHIVE_DEST_n. Am I correct?

Yes. Every standby database must have different db_unique_name otherwise you someone will be in mess.

1. There is a difference. The standby controlfile has, internally, an atribute set to define it as a standby controlfile. That attribute is set when you execute an "ALTER DATABASE CREATE STANDBY CONTROLFILE AS 'location'; "
Similarly, when you query V$DATABASE for "DATABASE_ROLE", it determines the database role based on the attribute in the controlfile.

2. Archival at the standby site is from the standby redo logs if you are using SRLs.

3. The Primary identifies the standbys based on LOG_ARCHIVE_DEST_n SERVICE_NAME. However, it is good practice to define a different DB_UNIQUE_NAME for each standby.